1. Field of the Invention
The present invention relates to a method and a system for suppressing formation of scale in a hot finishing mill system during rolling of a strip material.
2. Description of Related Art
When iron contacts a gas, such as oxygen or air, at a high temperature during rolling of a strip material, a film of the reaction product, i.e., scale, is formed on the surface of the strip material. This scale may exert an adverse influence, such as oxidation, on the strip material, and should be removed. The customary practice for removing scale formed on a strip material has been to spray a jet of pressurized water on the surface of the strip material.
FIG. 6, for example, is a schematic view of a scale removing device of a conventional hot finishing mill system.
In a conventional hot finishing mill system as shown in FIG. 6, a plurality of finishing mills, i.e., 1st to 7th finishing mills 101, 102, 103, 104, 105, 106, and 107, are located in a row along the direction of transport of a rolled material S downstream of a roughing mill (not shown) in the direction of transport. The finishing mills 101, 102, 103, 104, 105, 106, and 107 have a pair of (i.e., upper and lower) work rollers 201, 202, 203, 204, 205, 206, and 207, respectively. A finishing mill group 100 is constructed in this manner. On the entry side of the finishing mill group 100, a scale breaker 301 is provided for removing scale formed on the rolled material S. The scale breaker 301 has jet nozzles 302 above and below the rolled material S. These jet nozzles 302 direct jets of water at a high pressure of, say, 200 kgf/cm.sup.2, to the upper and lower surfaces of the rolled material S to remove the scale.
Thus, the rolled material S transported after rough rolling from a slab by a roughing mill is conveyed to the entry side of the finishing mill group 100 where scale formed on the surfaces of the rolled material S is removed by the scale breaker 301 before finish rolling. In detail, water pressurized at, say, 200 kgf/cm.sup.2, is directed through the upper and lower jet nozzles 302 to the upper and lower surfaces of the conveyed rolled material S to remove the adhering scale. The descaled rolled material S is carried to the finishing mill group 100 for rolling by the work rollers 201, 202, 203, 204, 205, 206, and 207 of the 1st to 7th finishing mills 101, 102, 103, 104, 105, 106, and 107, whereby it is sequentially finish rolled to predetermined thicknesses.
FIG. 3 is a graph illustrative of the relationship of a rolling load and the thickness of scale during descaling and finish rolling of the rolled material S. In this graph, the circle .largecircle. signifies the rolling load of the scale removing device of the aforementioned conventional hot finishing mill system, the two-dot chain line represents the thickness of scale, A designates the time of scale removal by the scale breaker 301, and B, C, D, E, F, G, and H represent the times of finish rolling by the 1st to 7th finishing mills 101, 102, 103, 104, 105, 106, and 107, respectively. This graph shows that the scale thickness of the rolled material S decreases rapidly at the time A of scale removal by the scale breaker 301, and also decreases at the times B, C, D, E, F, G and H of finish rolling. Also, repeated rolling is found to reduce the thickness of the scale.
With such a hot finishing mill system, there is a demand for transporting the rolled material S at a high speed in order to raise the work efficiency. When the rolled material S is transported at a high speed, however, its front end collides with the outer peripheral surfaces of the work rollers 201, 202, 203, 204, 205, 206, and 207 when engaged into the finishing mills 101, 102, 103, 104, 105, 106, and 107. As a result, the work rolls 201, 202, 203, 204, 205, 206, and 207 can be deformed or damaged. With the hot finishing mill system, therefore, the rolled material S has to be transported at a low speed, with the result that the rolled material S takes a relatively long time until its engagement into the work rolls 201, 202, 203, 204, 205, 206, and 207, thus promoting the formation of scale. Under this scenario, the thickness of the scale on the rolled material S after rolling exceeds a limit of 5 .mu.m. During finish rolling, this scale is imprinted into the surface of the rolled material S, causing defects. This markedly deteriorates the quality of the rolled material S.